fix: stored I tetrominos read as empty.

src/tetris.c

@@ -75,6 +75,7 @@ void tetris_place(u8 *restrict *restrict rows, u8 tetromino, uint x, uint y) {
 	uint *end = pos + 8;
 	tetris_get_blocks(tetromino, pos);
 	tetromino &= 7;    /* strip rotation data */
+	tetromino |= 0x80; /* set an undefined bit, to signify there is data here. */
 	for (; pos < end; pos += 2)
 		rows[*(pos + 1)][*pos] = tetromino;
 }